Plasma generation and processing with multiple radiation sources

ABSTRACT

Plasma-assisted methods and apparatus that use multiple radiation sources are provided. In one embodiment, a plasma is ignited by subjecting a gas in a radiation cavity to electromagnetic radiation having a frequency less than about 333 GHz in the presence of a plasma catalyst, which may be passive or active. A controller can be used to delay activation of one radiation source with respect to another radiation source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/430,415, filed May 7, 2003. Priority is claimed to U.S. ProvisionalPatent Application No. 60/378,693, filed May 8, 2002, No. 60/430,677,filed Dec. 4, 2002, and No. 60/435,278, filed Dec. 23, 2002, all ofwhich are fully incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to methods and apparatus for plasma-assistedprocessing, and in particular to using multiple electromagneticradiation sources in combination.

BACKGROUND OF THE INVENTION

It is known that a single microwave radiation source can be used togenerate a plasma by subjecting a gas to a sufficient amount ofmicrowave radiation. A single microwave energy source can be damaged,however, when one source directs microwave energy into a plasma chamberthat reflects the energy back toward the same source. This can beespecially problematic when there is no strong microwave absorber in thecavity, such as when a plasma is not yet formed. Also, multiplemicrowave energy sources are particularly susceptible to damage whencombined to ignite or sustain a plasma. For example, a first source canbe damaged when it directs microwave energy into a chamber because thatenergy could be directed into another simultaneously connected radiationsource.

It is also known that plasma ignition is usually easier at gas pressuressubstantially less than atmospheric pressure. However, vacuum equipment,which is required to lower the gas pressure, can be expensive, as wellas slow and energy-consuming. Moreover, the use of such equipment canlimit manufacturing flexibility.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION

Consistent with the present invention, apparatus and methods usingmultiple radiation sources (e.g., microwave radiation sources) areprovided. Plasmas formed from gases at pressures at about one atmosphereor higher can strongly absorb microwave radiation. Strong absorption canbe used to reduce the possibility of damage to a particular source byits own radiation that may be reflected back or radiation from othersources. Consequently, high power plasma-assisted processes can becarried out using multiple (e.g., low power) sources coupled to the sameplasma.

In one embodiment, a radiation apparatus may include a cavity. Theradiation apparatus can also include a first high-frequency radiationsource and a second high-frequency radiation source for directingradiation into the cavity. The radiation apparatus may include acontroller for sequentially activating the second radiation source afterthe first radiation source is activated.

In another embodiment consistent with this invention, a plasma furnacemay include a chamber, a conduit for supplying gas to the chamber, aplurality of radiation sources arranged to radiate radiation into thechamber, and a controller for delaying activation of all but a first ofthe plurality of radiation sources until after the first radiationsource is activated. A radiation apparatus and a plasma furnaceconsistent with this invention may include a plasma catalyst locatedproximate to the cavity. The plasma catalyst can cooperate with themicrowave radiation in the presence of a gas to form a plasma. Thecatalyst can be passive or active. A passive plasma catalyst can includeany object capable of inducing a plasma by deforming a local electricfield (e.g., an electromagnetic field) consistent with this invention,without necessarily adding additional energy. An active plasma catalyst,on the other hand, is any particle or high energy wave packet capable oftransferring a sufficient amount of energy to a gaseous atom or moleculeto remove at least one electron from the gaseous atom or molecule in thepresence of electromagnetic radiation. In both cases, a plasma catalystcan improve, or relax, the environmental conditions required to ignite aplasma.

In another embodiment consistent with this invention, a method forforming a plasma is provided. The method can include employing at leastfirst and second radiation sources arranged to direct radiation into aprocessing or heating region. The method can include introducing gasinto the region, activating the first radiation source to facilitateformation of plasma in the heating region, and activating the secondradiation source after the plasma is formed.

In yet another embodiment consistent with this invention, additionalmethods and apparatus are provided for forming a plasma using adual-cavity system. The system can include a first ignition cavity and asecond cavity in fluid communication with each other. The method caninclude (i) subjecting a gas in the first ignition cavity toelectromagnetic radiation having a frequency less than about 333 GHz,such that the plasma in the first cavity causes a second plasma to formin the second cavity, and (ii) sustaining the second plasma in thesecond cavity by subjecting it to additional electromagnetic radiation.

Additional plasma catalysts, and methods and apparatus for igniting,modulating, and sustaining a plasma consistent with this invention areprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the invention will be apparent upon consideration ofthe following detailed description, taken in conjunction with theaccompanying drawings, in which like reference characters refer to likeparts throughout, and in which:

FIG. 1A shows a schematic diagram of an illustrative apparatus thatincludes multiple radiation sources consistent with this invention;

FIG. 1B shows a flow chart for an illustrative method consistent withthis invention;

FIG. 2 shows a simplified illustrative embodiment of a portion of aplasma system for adding a powder plasma catalyst to a plasma cavity forigniting, modulating, or sustaining a plasma in a cavity consistent withthis invention;

FIG. 3 shows an illustrative plasma catalyst fiber with at least onecomponent having a concentration gradient along its length consistentwith this invention;

FIG. 4 shows an illustrative plasma catalyst fiber with multiplecomponents at a ratio that varies along its length consistent with thisinvention;

FIG. 5A shows another illustrative plasma catalyst fiber that includes acore underlayer and a coating consistent with this invention;

FIG. 5B shows a cross-sectional view of the plasma catalyst fiber ofFIG. 5A, taken from line 5B-5B of FIG. 5A, consistent with thisinvention;

FIG. 6 shows an illustrative embodiment of another portion of a plasmasystem including an elongated plasma catalyst that extends through anignition port consistent with this invention;

FIG. 7 shows an illustrative embodiment of an elongated plasma catalystthat can be used in the system of FIG. 6 consistent with this invention;

FIG. 8 shows another illustrative embodiment of an elongated plasmacatalyst that can be used in the system of FIG. 6 consistent with thisinvention; and

FIG. 9 shows an illustrative embodiment of a portion of a plasma systemfor directing ionizing radiation into a radiation chamber consistentwith this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Consistent with the present invention, plasma apparatus and methods thatuse multiple radiation sources are provided. In one embodiment, as shownin FIG. 1A, a radiation apparatus may include cavity 12. Further, in oneembodiment, the radiation apparatus may further include a plasmacatalyst located proximate to the cavity, which may cooperate with theradiation to cause the gas to form a plasma.

This invention may further relate to methods and apparatus forinitiating, modulating, and sustaining a plasma for a variety ofapplications, including heat-treating, synthesizing and depositingcarbides, nitrides, borides, oxides, and other materials, doping,carburizing, nitriding, and carbonitriding, sintering, multi-partprocessing, joining, sintering, decrystallizing, making and operatingfurnaces, gas exhaust-treating, waste-treating, incinerating, scrubbing,ashing, growing carbon structures, generating hydrogen and other gases,forming electrodeless plasma jets, plasma processing in assembly lines,sterilizing, etc.

In another embodiment, a plasma furnace is provided that may include achamber, a conduit for supplying gas to the chamber, a plurality ofradiation sources arranged to radiate radiation into the chamber and acontroller for delaying activation of all but a first of the pluralityof radiation sources until after the first radiation source isactivated. Each of these components is explained more fully below.

This invention can be used for controllably generating heat and forplasma-assisted processing to lower energy costs and increaseheat-treatment efficiency and plasma-assisted manufacturing flexibility.

Therefore, a plasma catalyst for initiating, modulating, and sustaininga plasma is provided. The catalyst can be passive or active. A passiveplasma catalyst can include any object capable of inducing a plasma bydeforming a local electric field (e.g., an electro-magnetic field)consistent with this invention without necessarily adding additionalenergy through the catalyst, such as by applying a voltage to create aspark. An active plasma catalyst, on the other hand, may be any particleor high energy wave packet capable of transferring a sufficient amountof energy to a gaseous atom or ion to remove at least one electron fromthe gaseous atom or molecule, in the presence of electromagneticradiation.

The following commonly owned U.S. patent applications are herebyincorporated by reference in their entireties: U.S. application Ser. No.10/513,221 (filed from PCT Application No. PCT/US03/14037), U.S.application Ser. No. 10/513,393 (filed from PCT Application No.PCT/US03/14124), PCT Application No. PCT/US03/14132, U.S. applicationSer. No. 10/513,394 (filed from PCT Application No. PCT/US03/14052),U.S. application Ser. No. 10/513,607 (filed from PCT Application No.PCT/US03/14036), U.S. application Ser. No. 10/449,600 (PCT ApplicationNo. PCT/US03/14133), application Ser. No. 10/430,414 (PCT ApplicationNo. PCT/US03/14053), PCT Application No. PCT/US03/14034, U.S.application Ser. No. 10/430,416 (PCT Application No. PCT/US03/14039),U.S. application Ser. No. 10/430,415 (PCT Application No.PCT/US03/14038), PCT Application No. PCT/US03/14133, U.S. applicationSer. No. 10/513,606 (filed from PCT Application No. PCT/US03/14035),U.S. application Ser. No. 10/513,309 (filed from PCT Application No.PCT/US03/14040), U.S. application Ser. No. 10/413,220 (filed from PCTApplication No. PCT/US03/14134), PCT Application No. PCT/US03/14122,U.S. application Ser. No. 10/513,397 (filed from PCT Application No.PCT/US03/14130), U.S. application Ser. No. 10/513,605 (filed from PCTApplication No. PCT/US03/14055), PCT Application No. PCT/US03/14137,U.S. application Ser. No. 10/430,426 (PCT Application No.PCT/US03/14123), PCT Application No. PCT/US03/14121, U.S. applicationSer. No. 10/513,604 (filed from PCT Application No. PCT/US03/14136), andPCT Application No. PCT/US03/14135).

Illustrative Plasma System

FIG. 1A shows a schematic diagram of an illustrative radiation apparatusconsistent with one aspect of the invention. The exemplary radiationapparatus may include cavity 12 formed in a vessel that may bepositioned inside a microwave chamber (also known as applicator) 14. Inanother embodiment (not shown), vessel 12 and microwave chamber 14 arethe same, thereby eliminating the need for two separate components. Thevessel in which cavity 12 is formed can include one or moreradiation-transmissive insulating layers to improve its thermalinsulation properties without significantly shielding cavity 12 from theradiation.

In one embodiment, the radiation apparatus may be configured as a plasmafurnace. One skilled in the art will appreciate that the radiationapparatus may also be used for initiating, modulating, and sustaining aplasma for a variety of other applications, including, for example,heat-treating, synthesizing and depositing carbides, nitrides, borides,oxides, and other materials, doping, carburizing, nitriding, andcarbonitriding, sintering, multi-part processing, joining, sintering,decrystallizing, making and operating furnaces, gas exhaust-treating,waste-treating, incinerating, scrubbing, ashing, growing carbonstructures, generating hydrogen and other gases, forming electrodelessplasma jets, plasma processing in assembly lines, sterilizing, etc.

In one embodiment, cavity 12 is formed in a vessel made of ceramic. Dueto the extremely high temperatures that can be achieved with plasmasconsistent with this invention, a ceramic capable of operating at about3,000 degrees Fahrenheit can be used. The ceramic material can include,by weight, 29.8% silica, 68.2% alumina, 0.4% ferric oxide, 1% titania,0.1% lime, 0.1% magnesia, 0.4% alkalies, which is sold under Model No.LW-30 by New Castle Refractories Company, of New Castle, Pa. It will beappreciated by those of ordinary skill in the art, however, that othermaterials, such as quartz, and those different from the one describedabove, can also be used consistent with the invention.

A plasma may be formed in a partially open cavity inside a first brickand topped with a second brick. The cavity may have dimensions of about2 inches by about 2 inches by about 1.5 inches. At least two holes mayalso be provided in the brick in communication with the cavity: one forviewing the plasma and at least one hole for providing the gas. The sizeof the cavity can depend on the desired plasma process being performed.Also, the cavity should at least be configured to prevent the plasmafrom rising/floating away from the primary processing region.

Cavity 12 can be connected to one or more gas sources 24 (e.g., a sourceof argon, nitrogen, hydrogen, xenon, krypton) by line 20 and controlvalve 22, which may be powered by power supply 28. Line 20 may be tubing(e.g., between about 1/16 inch and about ¼ inch, such as about ⅛″).Also, if desired, a vacuum pump can be connected to the chamber toremove any acidic fumes that may be generated during plasma processing.Also, any excess gas may escape the chamber via gas port 13 or similaradditional gas ports.

A radiation leak detector (not shown) may be installed near source 26and waveguide 30 and connected to a safety interlock system toautomatically turn off the radiation (e.g., microwave) power supply if aleak above a predefined safety limit, such as one specified by the FCCand/or OSHA (e.g., 5 mW/cm²), was detected.

In one embodiment, the radiation apparatus may include radiation source26 for directing radiation into the cavity. The radiation apparatus mayfurther include radiation source 27 for directing radiation into thecavity. Although FIG. 1A depicts two radiation sources, it will beappreciated that the radiation apparatus can operate with two or moresources.

In one embodiment, source 26 may be configured to generate radiationthat is cross-polarized relative to microwave radiation generated bysource 27 and/or any other additional sources.

Each of radiation sources 26 and 27 may be a magnetron, a klystron, agyrotron, a traveling-wave tube amplifier/oscillator or any other devicecapable of generating radiation, such as microwave radiation. Also, thefrequency of the radiation is believed to be non-critical in manyapplications. Thus, for example, radiation having any frequency lessthan about 333 GHz can be used consistent with this invention. Forexample, frequencies, such as power line frequencies (about 50 Hz toabout 60 Hz), can be used, although the pressure of the gas from whichthe plasma is formed may be lowered to assist with plasma ignition.Also, any radio frequency or microwave frequency can be used consistentwith this invention, including frequencies greater than about 100 kHz.In most cases, the gas pressure for such relatively high frequenciesneed not be lowered to ignite, modulate, or sustain a plasma, therebyenabling many plasma-processes to occur over a broad range of pressures,including atmospheric pressure and above.

Radiation source 26, which may be powered by electrical power supply 28,directs radiation energy into chamber 14 through one or more waveguides30. It will be appreciated by those of ordinary skill in the art thatsource 26 can be connected directly to cavity 12 or chamber 14, therebyeliminating waveguide 30. The radiation energy entering cavity 12 isused to ignite a plasma within the cavity. This plasma can besubstantially sustained and confined to the cavity by couplingadditional radiation with the catalyst. Other radiation sources (such as27) may similarly be directly connected to cavity 12 or chamber 14 orthrough one or more waveguides. Additionally, each one of them may bepowered by power supply 28 or any other combination of power suppliesmay be used.

Radiation energy, from radiation source 26, can be supplied throughcirculator 32 and tuner 34 (e.g., 3-stub tuner). Tuner 34 can be used tominimize the reflected power as a function of changing ignition orprocessing conditions, especially after the plasma has formed becausemicrowave power, for example, will be strongly absorbed by the plasma.Similarly, radiation energy from radiation source 27 may be suppliedthrough circulator 31 and tuner 33, although the use of circulators andtuners are optional.

In one embodiment, each of the radiation sources may be protectivelyseparated from the chamber by an isolator (not shown). An isolatorpermits radiation to pass in one direction only, thereby protecting asource not only from reflected radiation, but also from radiation fromother sources. However, consistent with this invention, reflectedradiation can be minimized, especially during the early stages of plasmaignition.

Detector 42 can develop output signals as a function of the temperatureor any other monitorable condition associated with a work piece (notshown) within cavity 12 and provide the signals to controller 44. Dualtemperature sensing and heating, as well as automated cooling and gasflow controls can also be used. Also, controller 44 may be programmed tosequentially activate source 27 after source 26 is activated. In anotherembodiment, controller 44 may delay activation of source 27 for apredetermined period following the activation of source 26. Sources 26and 27 may also be delayed in a similar manner and may be triggered byany measurable event, if desired.

In one embodiment, detector 42 may provide an indication of microwaveradiation absorption, and controller 44 may delay activation of one ormore of the plurality of microwave radiation sources until aftercontroller 44 receives a signal from detector 42 that a predeterminedabsorption threshold level has been reached.

Detector 42 may be any device that detects one or more of heat,radiation absorption, radiation reflectance, radiation transmission, theexistence of plasma, or any other phenomena signaling whether plasmaformation has or has not occurred. Examples of such detectors includeheat sensors, pyrometers, or any other sensor capable of detecting heat,temperature, radiation absorption, radiation reflectance, radiationtransmission, the existence of plasma, or any other radiation relatedphenomena.

Detector 42 can develop output signals as a function of the temperatureor any other monitorable condition associated with a work piece (notshown) within cavity 12 and provide the signals to controller 44. Dualtemperature sensing and heating, as well as automated cooling rate andgas flow controls can also be used. Controller 44 in turn can be used tocontrol operation of power supply 28, which can have one outputconnected to source 26 as described above and another output connectedto valve 22 to control gas flow into cavity 12. Although not shown,controller 44 or other similar controllers may be used to controloperation of any other power supplies that may be used to supply powerto the other radiation sources.

In another embodiment, detector 42 may provide an indication ofradiation absorption by, for example, an object that is being processed.In this case, controller 44 may delay activation of subsequent sourcesuntil after controller 44 receives a signal from detector 42 that apredetermined absorption level has been reached.

Controller 44 may also be configured to delay activation of at least oneof the plurality of radiation sources for a predetermined periodfollowing activation of the first radiation source. Then, each of theremaining plurality of radiation sources may be successively activatedat predetermined intervals, if desired. Controller 44 may also beconfigured to activate one or more additional radiation sources onlyafter at least one of the first and second radiation sources isactivated. Also, controller 44 may be configured to activate each of theplurality of additional radiation sources only after each of the firstand the second radiation sources is activated.

Consistent with this invention, a plasma apparatus may include a plasmacatalyst that is located proximate to a plasma cavity. The catalyst cancooperate with the radiation to cause a gas to form a plasma. Also, asused herein, the phrase “proximate the cavity” means either within thecavity or at a location sufficiently close to the cavity to effect theformation of the plasma.

As explained more fully below, the location of radiation-transmissivecavity 12 in chamber 14 may not be critical if chamber 14 supportsmultiple modes, and especially when the modes are continually orperiodically mixed. As also explained more fully below, motor 36 can beconnected to mode-mixer 38 for making the time-averaged radiation energydistribution substantially uniform throughout chamber 14. Furthermore,window 40 (e.g., a quartz window) can be disposed in one wall of chamber14 adjacent to cavity 12, permitting temperature detector 42 (e.g., anoptical pyrometer) to be used to view a process inside cavity 12. In oneembodiment, the optical pyrometer output can increase from zero volts asthe temperature rises to within the tracking range.

The invention may be practiced, for example, by employing microwavesources at both 915 MHz and 2.45 GHz provided by Communications andPower Industries (CPI), although radiation having any frequency lessthan about 333 GHz can be used. The 2.45 GHz system may provide, forexample, continuously variable microwave power from about 0.5 kilowattsto about 5.0 kilowatts. A 3-stub tuner may allow impedance matching formaximum power transfer and a dual directional coupler may be used tomeasure forward and reflected powers. Also, optical pyrometers may beused for remote sensing of the sample temperature.

As mentioned above, radiation having any frequency less than about 333GHz can be used consistent with this invention. For example,frequencies, such as power line frequencies (about 50 Hz to about 60Hz), can be used, although the pressure of the gas from which the plasmais formed may be lowered to assist with plasma ignition. Also, any radiofrequency or microwave frequency can be used consistent with thisinvention, including frequencies greater than about 100 kHz. In mostcases, the gas pressure for such relatively high frequencies need not belowered to ignite, modulate, or sustain a plasma, thereby enabling manyplasma-processes to occur at atmospheric pressures and above.

The equipment may be computer controlled using LabView 6i software,which may provide real-time temperature monitoring and microwave powercontrol. Noise may be reduced by using shift registers to generatesliding averages of suitable number of data points. Also, the number ofstored data points in the array may be limited to improve speed andcomputational efficiency. The pyrometer may measure the temperature of asensitive area of about 1 cm², which may be used to calculate an averagetemperature. The pyrometer may sense radiant intensities at twowavelengths and fit those intensities using Planck's law to determinethe temperature. It will be appreciated, however, that other devices andmethods for monitoring and controlling temperature are also availableand can be used consistent with this invention. Control software thatcan be used consistent with this invention is described, for example, incommonly owned, concurrently filed U.S. patent application Ser. No.10/___,___ (Attorney Docket No. 1837.0033), which is hereby incorporatedby reference in its entirety.

Chamber 14 may have several glass-covered viewing ports with radiationshields and one quartz window for pyrometer access. Several ports forconnection to a vacuum pump and a gas source may also be provided,although not necessarily used.

The exemplary radiation apparatus may also include a closed-loopdeionized water cooling system (not shown) with an external heatexchanger cooled by tap water. During operation, the deionized water mayfirst cool the magnetron, then the load-dump in the circulator (used toprotect the magnetron), and finally the microwave chamber through waterchannels welded on the outer surface of the chamber.

Methods and Apparatus Using Multiple Radiation Sources

FIG. 1B shows a method employing at least a first and second radiationsource, both arranged to direct radiation into a plasma formationregion. The method may include introducing a gas (step 45) into aplasma-formation region. In one embodiment, this may be accomplished byturning on valve 22 of FIG. 1A. It will be appreciated by those ofordinary skill in the art that the plasma-formation region could be acavity, which can be completely closed or partially open. For example,in certain applications, such as in plasma-assisted furnaces, the cavitycould be entirely closed. See, for example, commonly owned PCTApplication No. PCT/US03/14133, which is fully incorporated herein byreference. In other applications, however, it may be desirable to flow agas through the cavity, and therefore the cavity must be open to somedegree. In this way, the flow, type, and pressure of the flowing gas canbe varied over time. This may be desirable because certain gases withlower ionization potentials, such as argon, are easier to ignite but mayhave other undesirable properties during subsequent plasma processing.

The method may further include activating a first radiation source tofacilitate formation of plasma in step 47. In one embodiment, plasmaformation may be facilitated using some kind of a plasma catalyst, suchas a pointed metal tip, a spark generator, carbon, fibrous material,powderous material or any other catalyst capable of facilitating plasmaignition. Additional examples of plasma catalysts and their usesconsistent with the present invention are more fully described below.

The method may further include activating a second radiation sourceafter the plasma is formed (step 49). In one embodiment, radiationsource 27 may be activated after the first radiation source isactivated. The method may further include activating at least oneadditional radiation source after at least one of the first and secondsources is activated. Further, the activation of the at least oneadditional radiation source may be delayed until after both the firstand second sources are activated.

The radiation sources may be, for example, a magnetron, a klystron, agyrotron, a traveling-wave tube amplifier/oscillator, or any othersource of radiation. Further, in one embodiment, the radiation sourcesmay be cross-polarized.

Additionally, in one embodiment, the method may include activating aplurality of radiation sources, wherein each of the plurality ofmicrowave sources is successively activated at predetermined intervals.

In another embodiment, the plasma region may contain a plasma catalyst.The plasma catalyst consistent with this invention can include one ormore different materials and may be either passive or active. A plasmacatalyst can be used, among other things, to ignite, modulate, and/orsustain a plasma at a gas pressure that is less than, equal to, orgreater than atmospheric pressure.

Plasma Catalysts

One method of forming a plasma consistent with this invention caninclude subjecting a gas in a cavity to electromagnetic radiation havinga frequency less than about 333 GHz in the presence of a passive plasmacatalyst. A passive plasma catalyst consistent with this invention caninclude any object capable of inducing a plasma by deforming a localelectric field (e.g., an electromagnetic field) consistent with thisinvention, without necessarily adding additional energy through thecatalyst, such as by applying an electric voltage to create a spark.

A passive plasma catalyst consistent with this invention can also be anano-particle or a nano-tube. As used herein, the term “nano-particle”can include any particle having a maximum physical dimension less thanabout 100 nm that is at least electrically semi-conductive. Also, bothsingle-walled and multi-walled carbon nanotubes, doped and undoped, canbe particularly effective for igniting plasmas consistent with thisinvention because of their exceptional electrical conductivity andelongated shape. The nanotubes can have any convenient length and can bea powder fixed to a substrate. If fixed, the nanotubes can be orientedrandomly on the surface of the substrate or fixed to the substrate(e.g., at some predetermined orientation) while the plasma is ignited orsustained.

A passive plasma catalyst can also be a powder consistent with thisinvention, and need not comprise nano-particles or nano-tubes. It can beformed, for example, from fibers, dust particles, flakes, sheets, etc.When in powder form, the catalyst can be suspended, at leasttemporarily, in a gas. By suspending the powder in the gas, the powdercan be quickly dispersed throughout the cavity and more easily consumed,if desired.

In one embodiment, the powder catalyst can be carried into the cavityand at least temporarily suspended with a carrier gas. The carrier gascan be the same or different from the gas that forms the plasma. Also,the powder can be added to the gas prior to being introduced to thecavity. For example, as shown in FIG. 2, radiation source 52 andradiation source 54 can supply radiation to radiation cavity 55, inwhich plasma cavity 60 is placed. Powder source 65 provides catalyticpowder 70 into gas stream 75. In an alternative embodiment, powder 70can be first added to cavity 60 in bulk (e.g., in a pile) and thendistributed in the cavity in any number of ways, including flowing a gasthrough or over the bulk powder. In addition, the powder can be added tothe gas for igniting, modulating, or sustaining a plasma by moving,conveying, drizzling, sprinkling, blowing, or otherwise, feeding thepowder into or within the cavity. Although FIG. 2 shows only tworadiation sources, additional radiation sources may be used. In oneembodiment consistent with this invention, microwave source 54 may beactivated and then, after a plasma is formed, microwave source 55 may beactivated.

In one experiment, a plasma was ignited in a cavity by placing a pile ofcarbon fiber powder in a copper pipe that extended into the cavity.Although sufficient radiation was directed into the cavity, the copperpipe shielded the powder from the radiation and no plasma ignition tookplace. However, once a carrier gas began flowing through the pipe,forcing the powder out of the pipe and into the cavity, and therebysubjecting the powder to the radiation, a plasma was nearlyinstantaneously ignited in the cavity. This permits a subsequentradiation source to be activated, which reduces the ramp-up timerequired to achieve, for example, very high temperature plasmas.

A powder plasma catalyst consistent with this invention can besubstantially non-combustible, thus it need not contain oxygen or burnin the presence of oxygen. Thus, as mentioned above, the catalyst caninclude a metal, carbon, a carbon-based alloy, a carbon-based composite,an electrically conductive polymer, a conductive silicone elastomer, apolymer nanocomposite, an organic-inorganic composite, and anycombination thereof.

Also, powder catalysts can be substantially uniformly distributed in theplasma cavity (e.g., when suspended in a gas), and plasma ignition canbe precisely controlled within the cavity. Uniform ignition can beimportant in certain applications, including those applicationsrequiring brief plasma exposures, such as in the form of one or morebursts. Still, a certain amount of time can be required for a powdercatalyst to distribute itself throughout a cavity, especially incomplicated, multi-chamber cavities. Therefore, consistent with anotheraspect of this invention, a powder catalyst can be introduced into thecavity through a plurality of ignition ports to more rapidly obtain amore uniform catalyst distribution therein (see below).

In addition to powder, a passive plasma catalyst consistent with thisinvention can include, for example, one or more microscopic ormacroscopic fibers, sheets, needles, threads, strands, filaments, yarns,twines, shavings, slivers, chips, woven fabrics, tape, whiskers, or anycombination thereof. In these cases, the plasma catalyst can have atleast one portion with one physical dimension substantially larger thananother physical dimension. For example, the ratio between at least twoorthogonal dimensions should be at least about 1:2, but could be greaterthan about 1:5, or even greater than about 1:10.

Thus, a passive plasma catalyst can include at least one portion ofmaterial that is relatively thin compared to its length. A bundle ofcatalysts (e.g., fibers) may also be used and can include, for example,a section of graphite tape. In one experiment, a section of tape havingapproximately thirty thousand strands of graphite fiber, each about 2-3microns in diameter, was successfully used. The number of fibers in andthe length of a bundle are not critical to igniting, modulating, orsustaining the plasma. For example, satisfactory results have beenobtained using a section of graphite tape about one-quarter inch long.One type of carbon fiber that has been successfully used consistent withthis invention is sold under the trademark Magnamite®, Model No.AS4C-GP3K, by the Hexcel Corporation, of Anderson, S.C. Also,silicon-carbide fibers have been successfully used.

A passive plasma catalyst consistent with another aspect of thisinvention can include one or more portions that are, for example,substantially spherical, annular, pyramidal, cubic, planar, cylindrical,rectangular or elongated.

The passive plasma catalysts discussed above include at least onematerial that is at least electrically semi-conductive. In oneembodiment, the material can be highly conductive. For example, apassive plasma catalyst consistent with this invention can include ametal, an inorganic material, carbon, a carbon-based alloy, acarbon-based composite, an electrically conductive polymer, a conductivesilicone elastomer, a polymer nanocomposite, an organic-inorganiccomposite, or any combination thereof. Some of the possible inorganicmaterials that can be included in the plasma catalyst include carbon,silicon carbide, molybdenum, platinum, tantalum, tungsten, carbonnitride, and aluminum, although other electrically conductive inorganicmaterials are believed to work just as well.

In addition to one or more electrically conductive materials, a passiveplasma catalyst consistent with this invention can include one or moreadditives (which need not be electrically conductive). As used herein,the additive can include any material that a user wishes to add to theplasma. For example, in doping semiconductors and other materials, oneor more dopants can be added to the plasma through the catalyst. See,e.g., commonly owned U.S. patent application Ser. No. 10/513,397, (filedfrom PCT Application No. PCT/US03/14130), which is hereby incorporatedby reference in its entirety. The catalyst can include the dopantitself, or it can include a precursor material that, upon decomposition,can form the dopant. Thus, the plasma catalyst can include one or moreadditives and one or more electrically conductive materials in anydesirable ratio, depending on the ultimate desired composition of theplasma and the process using the plasma.

The ratio of the electrically conductive components to the additives ina passive plasma catalyst can vary over time while being consumed. Forexample, during ignition, the plasma catalyst could desirably include arelatively large percentage of electrically conductive components toimprove the ignition conditions. On the other hand, if used whilesustaining the plasma, the catalyst could include a relatively largepercentage of additives. It will be appreciated by those of ordinaryskill in the art that the component ratio of the plasma catalyst used toignite and sustain the plasma could be the same.

A predetermined ratio profile can be used to simplify many plasmaprocesses. In many conventional plasma processes, the components withinthe plasma are added as necessary, but such addition normally requiresprogrammable equipment to add the components according to apredetermined schedule. However, consistent with this invention, theratio of components in the catalyst can be varied, and thus the ratio ofcomponents in the plasma itself can be automatically varied. That is,the ratio of components in the plasma at any particular time can dependon which of the catalyst portions is currently being consumed by theplasma. Thus, the catalyst component ratio can be different at differentlocations within the catalyst. And, the current ratio of components in aplasma can depend on the portions of the catalyst currently and/orpreviously consumed, especially when the flow rate of a gas passingthrough the plasma chamber is relatively slow.

A passive plasma catalyst consistent with this invention can behomogeneous, inhomogeneous, or graded. Also, the plasma catalystcomponent ratio can vary continuously or discontinuously throughout thecatalyst. For example, in FIG. 3, the ratio can vary smoothly forming agradient along a length of catalyst 100. Catalyst 100 can include astrand of material that includes a relatively low concentration of acomponent at section 105 and a continuously increasing concentrationtoward section 110.

Alternatively, as shown in FIG. 4, the ratio can vary discontinuously ineach portion of catalyst 120, which includes, for example, alternatingsections 125 and 130 having different concentrations. It will beappreciated that catalyst 120 can have more than two section types.Thus, the catalytic component ratio being consumed by the plasma canvary in any predetermined fashion. In one embodiment, when the plasma ismonitored and a particular additive is detected, further processing canbe automatically commenced or terminated.

Another way to vary the ratio of components in a sustained plasma is byintroducing multiple catalysts having different component ratios atdifferent times or different rates. For example, multiple catalysts canbe introduced at approximately the same location or at differentlocations within the cavity. When introduced at different locations, theplasma formed in the cavity can have a component concentration gradientdetermined by the locations of the various catalysts. Thus, an automatedsystem can include a device by which a consumable plasma catalyst ismechanically inserted before and/or during plasma igniting, modulating,and/or sustaining.

A passive plasma catalyst consistent with this invention can also becoated. In one embodiment, a catalyst can include a substantiallynon-electrically conductive coating deposited on the surface of asubstantially electrically conductive material. Alternatively, thecatalyst can include a substantially electrically conductive coatingdeposited on the surface of a substantially electrically non-conductivematerial. FIGS. 5A and 5B, for example, show fiber 140, which includesunderlayer 145 and coating 150. In one embodiment, a plasma catalystincluding a carbon core is coated with nickel to prevent oxidation ofthe carbon.

A single plasma catalyst can also include multiple coatings. If thecoatings are consumed during contact with the plasma, the coatings couldbe introduced into the plasma sequentially, from the outer coating tothe innermost coating, thereby creating a time-release mechanism. Thus,a coated plasma catalyst can include any number of materials, as long asa portion of the catalyst is at least electrically semi-conductive.

Consistent with another embodiment of this invention, a plasma catalystcan be located entirely within a radiation cavity to substantiallyreduce or prevent radiation energy leakage. In this way, the plasmacatalyst does not electrically or magnetically couple with the vesselcontaining the cavity or to any electrically conductive object outsidethe cavity. This prevents sparking at the ignition port and preventsradiation from leaking outside the cavity during the ignition andpossibly later if the plasma is sustained. In one embodiment, thecatalyst can be located at a tip of a substantially electricallynon-conductive extender that extends through an ignition port.

FIG. 6, for example, shows radiation chamber 160 in which plasma cavity165 is placed. Plasma catalyst 170 is elongated and extends throughignition port 175. As shown in FIG. 7, and consistent with thisinvention, catalyst 170 can include electrically conductive distalportion 180 (which is placed in chamber 160) and electricallynon-conductive portion 185 (which is placed substantially outsidechamber 160). This configuration prevents an electrical connection(e.g., sparking) between distal portion 180 and chamber 160.

In another embodiment, shown in FIG. 8, the catalyst can be formed froma plurality of electrically conductive segments 190 separated by andmechanically connected to a plurality of electrically non-conductivesegments 195. In this embodiment, the catalyst can extend through theignition port between a point inside the cavity and another pointoutside the cavity, but the electrically discontinuous profilesignificantly prevents sparking and energy leakage.

Active Plasma Catalyst

Another method of forming a plasma consistent with this inventionincludes subjecting a gas in a cavity to electromagnetic radiationhaving a frequency less than about 333 GHz in the presence of an activeplasma catalyst, which generates or includes at least one ionizingparticle.

An active plasma catalyst consistent with this invention can be anyparticle or high energy wave packet capable of transferring a sufficientamount of energy to a gaseous atom or molecule to remove at least oneelectron from the gaseous atom or molecule in the presence ofelectromagnetic radiation. Depending on the source, the ionizingparticles can be directed into the cavity in the form of a focused orcollimated beam, or they may be sprayed, spewed, sputtered, or otherwiseintroduced.

For example, FIG. 9 shows radiation source 200 and radiation source 202for directing radiation into radiation chamber 205. Plasma cavity 210 ispositioned inside of chamber 205 and may permit a gas to flowtherethrough via ports 215 and 216. Source 220 directs ionizingparticles 225 into cavity 210. Source 220 can be protected, for example,by a metallic screen which allows the ionizing particles to pass throughbut shields source 220 from radiation. If necessary, source 220 can bewater-cooled. In one embodiment, radiation source 200 may be activatedand then, after a plasma is formed, radiation source 202 may beactivated. Alternatively, radiation source 202 may be activated after apredetermined interval.

Examples of ionizing particles consistent with this invention caninclude x-ray particles, gamma ray particles, alpha particles, betaparticles, neutrons, protons, and any combination thereof. Thus, anionizing particle catalyst can be charged (e.g., an ion from an ionsource) or uncharged and can be the product of a radioactive fissionprocess. In one embodiment, the vessel in which the plasma cavity isformed could be entirely or partially transmissive to the ionizingparticle catalyst. Thus, when a radioactive fission source is locatedoutside the cavity, the source can direct the fission products throughthe vessel to ignite the plasma. The radioactive fission source can belocated inside the radiation chamber to substantially prevent thefission products (i.e., the ionizing particle catalyst) from creating asafety hazard.

In another embodiment, the ionizing particle can be a free electron, butit need not be emitted in a radioactive decay process. For example, theelectron can be introduced into the cavity by energizing the electronsource (such as a metal), such that the electrons have sufficient energyto escape from the source. The electron source can be located inside thecavity, adjacent the cavity, or even in the cavity wall. It will beappreciated by those of ordinary skill in the art that any combinationof electron sources is possible. A common way to produce electrons is toheat a metal, and these electrons can be further accelerated by applyingan electric field.

In addition to electrons, free energetic protons can also be used tocatalyze a plasma. In one embodiment, a free proton can be generated byionizing hydrogen and, optionally, accelerated with an electric field.

Multi-mode Radiation Cavities

A radiation waveguide, cavity, or chamber can support or facilitatepropagation of at least one electromagnetic radiation mode. As usedherein, the term “mode” refers to a particular pattern of any standingor propagating electromagnetic wave that satisfies Maxwell's equationsand the applicable boundary conditions (e.g., of the cavity). In awaveguide or cavity, the mode can be any one of the various possiblepatterns of propagating or standing electromagnetic fields. Each mode ischaracterized by its frequency and polarization of the electric fieldand/or the magnetic field vectors. The electromagnetic field pattern ofa mode depends on the frequency, refractive indices or dielectricconstants, and waveguide or cavity geometry.

A transverse electric (TE) mode is one whose electric field vector isnormal to the direction of propagation. Similarly, a transverse magnetic(TM) mode is one whose magnetic field vector is normal to the directionof propagation. A transverse electric and magnetic (TEM) mode is onewhose electric and magnetic field vectors are both normal to thedirection of propagation. A hollow metallic waveguide does not typicallysupport a normal TEM mode of radiation propagation. Even thoughradiation appears to travel along the length of a waveguide, it may doso only by reflecting off the inner walls of the waveguide at someangle. Hence, depending upon the propagation mode, the radiation (e.g.,microwave) may have either some electric field component or somemagnetic field component along the axis of the waveguide (often referredto as the z-axis).

The actual field distribution inside a cavity or waveguide is asuperposition of the modes therein. Each of the modes can be identifiedwith one or more subscripts (e.g., TE₁₀ (“tee ee one zero”). Thesubscripts normally specify how many “half waves” at the guidewavelength are contained in the x and y directions. It will beappreciated by those skilled in the art that the guide wavelength can bedifferent from the free space wavelength because radiation propagatesinside the waveguide by reflecting at some angle from the inner walls ofthe waveguide. In some cases, a third subscript can be added to definethe number of half waves in the standing wave pattern along the z-axis.

For a given radiation frequency, the size of the waveguide can beselected to be small enough so that it can support a single propagationmode. In such a case, the system is called a single-mode system (i.e., asingle-mode applicator). The TE₁₀ mode is usually dominant in arectangular single-mode waveguide.

As the size of the waveguide (or the cavity to which the waveguide isconnected) increases, the waveguide or applicator can sometimes supportadditional higher order modes forming a multi-mode system. When manymodes are capable of being supported simultaneously, the system is oftenreferred to as highly moded.

A simple, single-mode system has a field distribution that includes atleast one maximum and/or minimum. The magnitude of a maximum largelydepends on the amount of radiation supplied to the system. Thus, thefield distribution of a single mode system is strongly varying andsubstantially non-uniform.

Unlike a single-mode cavity, a multi-mode cavity can support severalpropagation modes simultaneously, which, when superimposed, results in acomplex field distribution pattern. In such a pattern, the fields tendto spatially smear and, thus, the field distribution usually does notshow the same types of strong minima and maxima field values within thecavity. In addition, as explained more fully below, a mode-mixer can beused to “stir” or “redistribute” modes (e.g., by mechanical movement ofa radiation reflector). This redistribution desirably provides a moreuniform time-averaged field distribution within the cavity.

A multi-mode cavity consistent with this invention can support at leasttwo modes, and may support many more than two modes. Each mode has amaximum electric field vector. Although there may be two or more modes,one mode may be dominant and has a maximum electric field vectormagnitude that is larger than the other modes. As used herein, amulti-mode cavity may be any cavity in which the ratio between the firstand second mode magnitudes is less than about 1:10, or less than about1:5, or even less than about 1:2. It will be appreciated by those ofordinary skill in the art that the smaller the ratio, the moredistributed the electric field energy between the modes, and hence themore distributed the radiation energy is in the cavity.

The distribution of plasma within a processing cavity may stronglydepend on the distribution of the applied radiation. For example, in apure single mode system, there may only be a single location at whichthe electric field is a maximum. Therefore, a strong plasma may onlyform at that single location. In many applications, such a stronglylocalized plasma could undesirably lead to non-uniform plasma treatmentor heating (i.e., localized overheating and underheating).

Whether or not a single or multi-mode cavity is used consistent withthis invention, it will be appreciated by those of ordinary skill in theart that the cavity in which the plasma is formed can be completelyclosed or partially open. For example, in certain applications, such asin plasma-assisted furnaces, the cavity could be entirely closed. See,for example, commonly owned PCT Application No. PCT/US03/14133, which isfully incorporated herein by reference. In other applications, however,it may be desirable to flow a gas through the cavity, and therefore thecavity must be open to some degree. In this way, the flow, type, andpressure of the flowing gas can be varied over time. This may bedesirable because certain gases with lower ionization potentials, suchas argon, are easier to ignite but may have other undesirable propertiesduring subsequent plasma processing.

Mode-Mixing

For many applications, a cavity containing a uniform plasma isdesirable. However, because microwave radiation can have a relativelylong wavelength (e.g., several tens of centimeters), obtaining a uniformdistribution can be difficult to achieve. As a result, consistent withone aspect of this invention, the radiation modes in a multi-mode cavitycan be mixed, or redistributed, over a period of time. Because the fielddistribution within the cavity must satisfy all of the boundaryconditions set by the inner surface of the cavity, those fielddistributions can be changed by changing the position of any portion ofthat inner surface.

In one embodiment consistent with this invention, a movable reflectivesurface can be located inside the radiation cavity. The shape and motionof the reflective surface should, when combined, change the innersurface of the cavity during motion. For example, an “L” shaped metallicobject (i.e., “mode-mixer”) when rotated about any axis will change thelocation or the orientation of the reflective surfaces in the cavity andtherefore change the radiation distribution therein. Any otherasymmetrically shaped object can also be used (when rotated), butsymmetrically shaped objects can also work, as long as the relativemotion (e.g., rotation, translation, or a combination of both) causessome change in the location or the orientation of the reflectivesurfaces. In one embodiment, a mode-mixer can be a cylinder that isrotatable about an axis that is not the cylinder's longitudinal axis.

Each mode of a multi-mode cavity may have at least one maximum electricfield vector, but each of these vectors could occur periodically acrossthe inner dimension of the cavity. Normally, these maxima are fixed,assuming that the frequency of the radiation does not change. However,by moving a mode-mixer such that it interacts with the radiation, it ispossible to move the positions of the maxima. For example, mode-mixer 38can be used to optimize the field distribution within cavity 14 suchthat the plasma ignition conditions and/or the plasma sustainingconditions are optimized. Thus, once a plasma is excited, the positionof the mode-mixer can be changed to move the position of the maxima fora uniform time-averaged plasma process (e.g., heating).

Thus, consistent with this invention, mode-mixing can be useful duringplasma ignition. For example, when an electrically conductive fiber isused as a plasma catalyst, it is known that the fiber's orientation canstrongly affect the minimum plasma-ignition conditions. It has beenreported, for example, that when such a fiber is oriented at an anglethat is greater than 60° to the electric field, the catalyst does littleto improve, or relax, these conditions. By moving a reflective surfaceeither in or near the cavity, however, the electric field distributioncan be significantly changed.

Mode-mixing can also be achieved by launching the radiation into theapplicator chamber through, for example, a rotating waveguide joint thatcan be mounted inside the applicator chamber. The rotary joint can bemechanically moved (e.g., rotated) to effectively launch the radiationin different directions in the radiation chamber. As a result, achanging field pattern can be generated inside the applicator chamber.

Mode-mixing can also be achieved by launching radiation in the radiationchamber through a flexible waveguide. In one embodiment, the waveguidecan be mounted inside the chamber. In another embodiment, the waveguidecan extend into the chamber. The position of the end portion of theflexible waveguide can be continually or periodically moved (e.g., bent)in any suitable manner to launch the radiation (e.g., microwaveradiation) into the chamber at different directions and/or locations.This movement can also result in mode-mixing and facilitate more uniformplasma processing (e.g., heating) on a time-averaged basis.Alternatively, this movement can be used to optimize the location of aplasma for ignition or other plasma-assisted process.

If the flexible waveguide is rectangular, a simple twisting of the openend of the waveguide will rotate the orientation of the electric and themagnetic field vectors in the radiation inside the applicator chamber.Then, a periodic twisting of the waveguide can result in mode-mixing aswell as rotating the electric field, which can be used to assistignition, modulation, or sustaining of a plasma.

Thus, even if the initial orientation of the catalyst is perpendicularto the electric field, the redirection of the electric field vectors canchange the ineffective orientation to a more effective one. Thoseskilled in the art will appreciate that mode-mixing can be continuous,periodic, or preprogrammed.

In addition to plasma ignition, mode-mixing can be useful duringsubsequent plasma processing to reduce or create (e.g., tune) “hotspots” in the chamber. When a microwave cavity only supports a smallnumber of modes (e.g., less than 5), one or more localized electricfield maxima can lead to “hot spots” (e.g., within cavity 12). In oneembodiment, these hot spots could be configured to coincide with one ormore separate, but simultaneous, plasma ignitions or processing events.Thus, the plasma catalyst can be located at one or more of thoseignition or subsequent processing positions.

Multi-Location Ignition

A plasma can be ignited using multiple plasma catalysts at differentlocations. In one embodiment, multiple fibers can be used to ignite theplasma at different points within the cavity. Such multi-point ignitioncan be especially beneficial when a uniform plasma ignition is desired.For example, when a plasma is modulated at a high frequency (i.e., tensof Hertz and higher), or ignited in a relatively large volume, or both,substantially uniform instantaneous striking and restriking of theplasma can be improved. Alternatively, when plasma catalysts are used atmultiple points, they can be used to sequentially ignite a plasma atdifferent locations within a plasma chamber by selectively introducingthe catalyst at those different locations. In this way, a plasmaignition gradient can be controllably formed within the cavity, ifdesired.

Also, in a multi-mode cavity, random distribution of the catalystthroughout multiple locations in the cavity increases the likelihoodthat at least one of the fibers, or any other passive plasma catalystconsistent with this invention, is optimally oriented with the electricfield lines. Still, even where the catalyst is not optimally oriented(not substantially aligned with the electric field lines), the ignitionconditions are improved.

Furthermore, because a catalytic powder can be suspended in a gas, it isbelieved that each powder particle may have the effect of being placedat a different physical location within the cavity, thereby improvingignition uniformity within the cavity.

Dual-Cavity Plasma Igniting/Sustaining

A dual-cavity arrangement can be used to ignite and sustain a plasmaconsistent with this invention. In one embodiment, a system includes atleast a first ignition cavity and a second cavity in fluid communicationwith the first cavity. To ignite a plasma, a gas in the first ignitioncavity can be subjected to electromagnetic radiation having a frequencyless than about 333 GHz, optionally in the presence of a plasmacatalyst. In this way, the proximity of the first and second cavitiesmay permit a plasma formed in the first cavity to ignite a plasma in thesecond cavity, which may be sustained with additional electromagneticradiation.

In one embodiment of this invention, the first cavity can be very smalland designed primarily, or solely for plasma ignition. In this way, verylittle microwave energy may be required to ignite the plasma, permittingeasier ignition, especially when a plasma catalyst is used consistentwith this invention.

In one embodiment, the first cavity may be a substantially single modecavity and the second cavity is a multi-mode cavity. When the firstignition cavity only supports a single mode, the electric fielddistribution may strongly vary within the cavity, forming one or moreprecisely located electric field maxima. Such maxima are normally thefirst locations at which plasmas ignite, making them ideal points forplacing plasma catalysts. It will be appreciated, however, that when aplasma catalyst is used, it need not be placed in the electric fieldmaximum and, many cases, need not be oriented in any particulardirection.

In the foregoing described embodiments, in some instances, variousfeatures may be grouped together in a single embodiment for purposes ofstreamlining the disclosure. This method of disclosure is not to beinterpreted as reflecting an intention that the claimed inventionrequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby incorporated into this Detailed Description ofEmbodiments, with each claim standing on its own as a separate preferredembodiment of the invention.

1. A radiation apparatus, comprising: a radiation cavity; a firstradiation source for directing electromagnetic radiation having afrequency less than about 333 GHz into the radiation cavity tofacilitate formation of plasma in the radiation cavity; a secondradiation source having a frequency less than about 333 GHz fordirecting electromagnetic radiation into the radiation cavity; and acontroller for sequentially activating the second radiation source afterthe first radiation source is activated.
 2. The apparatus of claim 1,further comprising a detector that provides an indication ofelectromagnetic radiation absorption, and wherein the controller delaysactivation of the second radiation source until the controller receivesa signal from the detector that a predetermined absorption level hasbeen reached.
 3. The apparatus of claim 1 wherein the controlleractivates the second radiation source after the first radiation sourceforms the plasma.
 4. The apparatus of claim 1, wherein the firstradiation source and the second radiation source are activatedsimultaneously if the first radiation source is cross-polarized withrespect to the second radiation source.
 5. The apparatus of claim 1,wherein the apparatus further comprises a catalyst.
 6. The apparatus ofclaim 5 wherein the catalyst is located to reduce electromagneticradiation energy leakage.
 7. The apparatus of claim 5 wherein thecatalyst has an electrically discontinuous profile.
 8. The apparatus ofclaim 5 wherein the catalyst has a variable composition.
 9. Theapparatus of claim 5, wherein the catalyst is a nanostructure.
 10. Theapparatus of claim 1, further comprising a circulator for deliveringelectromagnetic radiation.
 11. The apparatus of claim 1, furthercomprising a tuner to process reflected power.
 12. The apparatus ofclaim 1, further comprising an electron source.
 13. The apparatus ofclaim 1, further comprising a mode mixer.
 14. The apparatus of claim 1,further comprising a flexible waveguide.
 15. The apparatus of claim 1,further comprising a second radiation cavity.
 16. The apparatus of claim15 wherein the second radiation cavity is substantially at leastsingle-mode.
 17. A plasma apparatus, comprising: a chamber; a conduitfor supplying a gas to the chamber; a plurality of radiation sourceshaving a frequency less than about 333 GHz arranged to deliverelectromagnetic radiation into the chamber; and a controller fordelaying activation of all but a first of the plurality of radiationsources until after the first of the plurality of radiation sources isactivated.
 18. The apparatus of claim 17, further comprising a detectorthat provides an indication of electromagnetic radiation absorption, andwherein the controller delays activation of each one of the plurality ofradiation sources until the controller receives a signal from thedetector that a predetermined absorption level has been reached.
 19. Theapparatus of claim 18 wherein the detector is connected to thecontroller.
 20. The apparatus of claim 17 wherein the controlleractivates a second radiation source of the plurality of radiationsources after the first of the plurality of radiation sources forms theplasma.
 21. The apparatus of claim 17, wherein at least two sources ofthe plurality of radiation sources are activated simultaneously if thereexists a cross-polarization between the at least two sources of theplurality of radiation sources.
 22. The apparatus of claim 17 whereinthe apparatus further comprises a catalyst.
 23. The apparatus of claim22 wherein the catalyst is located to reduce electromagnetic radiationenergy leakage.
 24. The apparatus of claim 22 wherein the catalyst hasan electrically discontinuous profile.
 25. The apparatus of claim 22wherein the catalyst has a variable composition.
 26. The apparatus ofclaim 17, further comprising a second radiation cavity.
 27. Theapparatus of claim 26 wherein the second radiation cavity is at leastsubstantially single-mode.
 28. The apparatus of claim 17 wherein thecatalyst is a nanostructure.
 29. The apparatus of claim 17, furthercomprising a circulator for delivering electromagnetic radiation. 30.The apparatus of claim 17, further comprising a tuner to processreflected power.
 31. The apparatus of claim 17, the chamber includes aradiation cavity.
 32. The apparatus of claim 17 wherein the chamber isconfigured to support at least one electromagnetic mode.
 33. Theapparatus of claim 17, further comprising a mode mixer.
 34. A methodemploying at least a first radiation source and a second radiationsource arranged to direct electromagnetic radiation into a plasmaregion, the method comprising: introducing a gas into the plasma region;activating the first radiation source having a frequency less than about333 GHz in order to facilitate formation of a plasma in the plasmaregion; and activating the second radiation source having a frequencyless than about 333 GHz after the plasma is formed.
 35. The method ofclaim 34, further including a step of starting the first radiationsource and the second radiation source simultaneously if the firstradiation source is cross-polarized with respect to the second radiationsource.
 36. The method of claim 34, wherein formation of the plasma isfacilitated using at least one catalyst capable of causing plasmaignition.
 37. The method of claim 36 wherein the at least one catalystincludes at least one dopant.
 38. The method of claim 36, furtherincluding a step of introducing the at least one catalyst at a variablespeed.
 39. The method of claim 34, further comprising a step ofactivating at least a third radiation source after one of the firstradiation source and the second radiation source is activated.
 40. Themethod of claim 34, further comprising a step of delaying activation ofthe at least a third radiation source until the first radiation sourceand the second radiation source are activated.
 41. The method of claim34, wherein electromagnetic radiation from the first radiation source iscross-polarized relative to electromagnetic radiation from the secondradiation source.
 42. The method of claim 34, further comprising a stepof activating a plurality of radiation sources, wherein each of theplurality of radiation sources is successively activated at at least onepredetermined interval.
 43. The method of claim 34, wherein the plasmaregion contains a plasma catalyst.
 44. The method of claim 34, whereinthe step of activating the first radiation source further comprises astep of: igniting the plasma by subjecting the gas in the plasma regionto electromagnetic radiation generated by the first radiation sourcehaving a frequency less than about 333 GHz in a presence of at least onepassive plasma catalyst comprising a material that is at leastelectrically semi-conductive.
 45. The method of claim 44, wherein thematerial is in the form of at least one of a nano-particle, a nano-tube,a powder, a dust, a flake, a fiber, a sheet, a needle, a thread, astrand, a filament, a yarn, a twine, a shaving, a sliver, a chip, awoven fabric, a tape, and a whisker.
 46. The method of claim 45, whereinthe method further includes a step of adjusting an orientation of thenanotube.
 47. The method of claim 44, wherein the at least one passiveplasma catalyst comprises a plurality of elongated, electricallyconductive items distributed in a plurality of locations in a radiationcavity enclosing the plasma region.
 48. The method of claim 44, whereinthe radiation cavity enclosing the plasma region is configured tosupport at least a first mode and a second mode of electromagneticradiation, each of the modes having a maximum electric field vector inthe radiation cavity, each of the electric field vectors having amagnitude, and wherein a ratio between the first mode magnitude and thesecond mode magnitude is less than about 1:10.
 49. The method of claim34, wherein the step of activating the first radiation source furthercomprises: subjecting the gas in the plasma region to electromagneticradiation having a frequency less than about 333 GHz in a presence of anactive plasma catalyst comprising at least one ionizing particle. 50.The method of claim 49, wherein the at least one ionizing particle is acharged particle.
 51. The method of claim 49, wherein the at least oneionizing particle comprises a radioactive fission product.
 52. Themethod of claim 51, wherein the radiation cavity enclosing the plasmaregion is formed in a vessel that is at least partially transmissive tothe radioactive fission product, the method further comprising a step ofpositioning a radioactive fission source outside the radiation cavityenclosing the plasma region such that the radioactive fission sourcedirects the radioactive fission product through the vessel into theradiation cavity enclosing the plasma region.
 53. The method of claim51, wherein the vessel and the radioactive fission source are inside achamber, and wherein the chamber comprises a material that substantiallyprevents the radioactive fission product from escaping the chamber. 54.The method of claim 51 further comprising a step of positioning aradioactive fission source in the radiation cavity enclosing the plasmaregion, wherein the radioactive fission source generates the at leastone radioactive fission product.
 55. The method of claim 34 furtherincluding a step of releasing a carbon fiber into the plasma region froma structure shielding the carbon fiber from electromagnetic radiation.56. The method of claim 34 further including a step of accomplishing apredetermined heating gradient using a mode-mixer.
 57. The method ofclaim 34 further including a step of mode mixing.